I repeatedly got the -2999 error when trying to download one particular library book. I got conflicting information from my web search on the error, so I couldn't fix it. I was able to retrieve the book when I tried again the next day.
I think it may have something to do with a timeout when Reader Library tries to verify the DRM with the server when it processes the .acsm file, but that's more of a guess than a real diagnosis.
If you can't download it later, contact
ereadersupport@waterstones.com for help.